VOR — VHF Omnidirectional Range
VOR has been since the 1950s the most important ground-based radio-nav system in civil aviation. A ground station transmits a signal from which the aircraft derives its magnetic bearing (radial) from the station.
Source: ICAO Annex 10 Vol I §3.
Operating principle — phase comparison
The VOR station transmits two signals on the same frequency:
- Reference signal (omnidirectional).
- Variable signal with a rotating phase pattern (phase varies linearly with magnetic bearing from the station).
The aircraft receiver determines the radial via the principle of phase comparison of two signals — the phase difference equals the magnetic bearing from the station (radial).
The radial (QDR) of a VOR station is related to magnetic north.
Frequency band
VHF omnidirectional beacons (VOR) operate in the frequency band 108 MHz to 117.975 MHz:
- 50 kHz channel spacing.
- 108.00–111.95 MHz: even 100-kHz channels for ILS localizer; odd 100-kHz channels for VOR.
- 112.00–117.95 MHz: VOR only.
Example: 110.1 MHz (VOR Frankfurt), 112.3 MHz (VOR Trasadingen).
Example frequency: a transmitter at 108.8 MHz is a VOR — 108.8 MHz is an odd 100-kHz frequency in the VOR band.
TVOR with ATIS — A9 modulation
A VOR on frequency 110.6 MHz/A9 is a TVOR (Terminal VOR) broadcasting ATIS:
- A9 modulation = combined A2/A3: VOR identifier (A2, Morse with audio) plus voice modulation (A3) for ATIS broadcast.
- Pilot can hear the ATIS directly from the VOR receiver, no extra COM frequency.
- Use: many terminal VORs (class T) at airports also broadcast ATIS on the VOR frequency.
Quasi-optical propagation
Transmissions of a VOR ground station can be subject to diffraction, absorption and reflection; its range is limited by the quasi-optical propagation of radio waves in the VHF band. Consequence: VOR is a line-of-sight beacon → no reception below the horizon.
Range — altitude-dependent
VOR range depends on altitude (rule of thumb R [NM] ≈ 1.23 × √h [ft]):
| Altitude AGL | VOR range |
|---|---|
| 1 000 ft | ~ 39 NM |
| 3 000 ft | ~ 67 NM |
| 5 000 ft | ~ 88 NM |
| 10 000 ft | ~ 123 NM |
| FL200 | ~ 174 NM |
At 5000 ft AGL, a range of about 88 NM in VHF (VOR) can be expected.
Classes (service volume, ICAO Annex 10)
| Class | Range | Altitude |
|---|---|---|
| Terminal (T) / TVOR | 25 NM | 1000–12 000 ft AGL |
| Low (L) | 40 NM | 1000–18 000 ft AGL |
| High (H) | 40 NM at low altitude, 130 NM at FL145–FL600 | layered |
Cockpit operation
OBS — course selector
The "OBS" button on the VOR indicator is the course selector — the pilot selects the desired radial.
CDI (Course Deviation Indicator)
- Needle shows deviation from selected OBS.
- Scale: typically 5 dots each side.
- Each dot equals 2° deviation in VOR.
- A full deflection of the CDI needle with VOR on means deviation from the reference course (OBS) of more than 10° (5 dots × 2° = 10° full scale).
TO/FROM — radial vs course
When the VOR indicator shows "TO" then the radial is read from the opposite side:
- "FROM" means the radial equals the OBS value (radial goes from the station through the aircraft).
- "TO" means the radial is opposite to the OBS (180° + OBS = current radial).
- Directly overhead the station the TO/FROM toggles to an "OFF" flag — equivalent to "station passage".
Worked example: position determination
An aircraft on north heading (000°), OBS at 180°, "TO" indication and CDI needle left of centre. Relative to VOR:
- OBS 180° + "TO" → actual radial = 360°/0° (pilot coming from south, flying north, station ahead).
- CDI left → station is left of pilot (i.e. west) → so pilot is east of station.
- Combination: pilot is northwest of station.
Accuracy
The approximate accuracy of a VOR is ± 2°.
- ICAO Annex 10 requirement: ≤ ±5° (95 % confidence).
- Often better in practice: ±1-2° standard VOR, ±0.5-1° DVOR.
- At 60 NM, 2° = 2 NM lateral accuracy.
Cone of silence — overhead the VOR
The term "cone of silence" means the funnel-shaped area directly overhead the transmission antenna within which no reception is possible:
- About 30° half-angle above the VOR — signal "disappears" on overflight.
- Immediately before the approached VOR station, the CDI deviates — because the aircraft approaches the cone of silence.
- Symptom: TO/FROM flag → "OFF" or flickering; CDI swings.
- Pilot knows he is overhead — used as station passage (radio fix).
VORTAC and VOR/DME
VOR types:
- VOR: VOR alone.
- VOR/DME: VOR + civilian DME co-located.
- VORTAC: VOR + military TACAN — TACAN contains a DME-equivalent distance signal.
Only the VOR component of a VORTAC ground station can be used with a conventional VOR receiver — the TACAN bearing signal is not decoded by civilian VOR receivers.
DVOR — Doppler VOR
Doppler VOR (DVOR) is a newer generation:
- Uses Doppler shift for bearing determination (via a ring antenna array).
- DVORs have better accuracy than normal VORs and can be used with every normal VOR receiver — no on-board hardware change.
- Less susceptible to ground reflections (mountains or buildings).
Advantages of VOR over NDB
Advantages of VOR navigation over NDB are lower susceptibility to atmospheric disruption and higher accuracy:
- Quasi-optical VHF propagation: no sky-wave interference, no strong night drift, no coast refraction comparable to NDB.
- Accuracy ±2° vs ±5-10° NDB.
- Phase comparison more robust than pure bearing antenna.
Cross-cut — fix with two VORs
Two VOR radials intersected give a position:
- Best accuracy at intersection 60°–120°.
- Smaller angle (e.g. < 30°) blows up the error ellipse.
Operational
- Pre-flight: verify frequency and identifier (3-letter Morse, repeated).
- In flight: check correct OBS regularly.
- Limitations: static near CB, spurious signals near reflecting obstacles (tunnels, mountains).
VOR phase-out
MON (Minimum Operating Network): FAA and EASA reduce the VOR network in favour of GNSS. As of 2024 many US VORs already off, Europe slower. Important — not all charted VORs are active! Check NOTAMs.